This invention relates to a method and apparatus for gasifying solid organic materials to convert the chemical energy stored in such materials to thermal energy. More particularly, this invention relates to a method and apparatus for gasifying biomass materials such as wood chips, sawdust, corn cobs, bagasse, tobacco waste and the like. The invention utilizes a new and improved feed hopper with inclined reciprocating plates lying along the bottom of the feed hopper to prevent the feed material from jamming in the hopper, even when the feed stock is moist and/or irregularly or nonuniformly shaped, and it utilizes a domed catalytic oxidation chamber where the feed stock is partially oxidized at an elevated temperature. The high temperature gases produced by the practice of the invention can be utilized to advantage, for example, as the thermal energy source for a conventional water tube boiler.